JP 2007-208472 A and JP 2008-92172 A disclose techniques related to an optical communication system and an optical transmitter. According to the techniques, an optical modulator and a driver (e.g. an electrical amplifier) which applies a drive signal to the optical modulator are operated in a saturated region as much as possible to obtain stable transmission light. For example, a drive signal having an amplitude corresponding to a double (2×Vπ) half-wavelength voltage Vπ is applied to an optical modulator for an optical QPSK (Quadrature Phase Shift Keying) scheme.
Meanwhile, in order to expand a transmission distance and transmission capacity in an optical communication system, it has been studied that a digital signal processing is applied to an optical transmitter in the optical communication system. For example, it is possible to generate signals with various formats such as a dispersion pre-equalization signal, a Nyquist signal and a quadrature amplitude modulation (QAM) signal are generated by using the digital signal processing. By applying any one of these signals as a drive signal to an optical modulator, it is possible to generate transmission light with the format according to the digital signal processing.
For example, by using the dispersion pre-equalization signal, it is possible to compensate for wavelength dispersion which occurs in transmission light propagating in an optical transmission line to expand a transmission distance of the transmission light. By using a Nyquist signal which is subjected to a band limitation with a Nyquist filter in the digital signal processing, it is possible to improve frequency usage efficiency according to the band limitation. By using a QAM signal, it is possible to transmit a greater amount of information than that of, for example, a phase-shift keying (PSK) signal to expand transmission capacity.
Thus, in a case where signal lights with various modulation formats are available by applying the digital signal processing to the optical transmitter, the optical modulator may be driven by a drive signal with an amplitude smaller than 2×Vπ. In other words, the optical modulator and the driver may be operated in a linear region.
In such a case, when characteristics of a drive signal are changed, characteristics of modulated signal light output from the optical modulator are also changed. Such change may cause a deterioration in quality of the modulated signal light. The above-mentioned techniques fail to study the possibility of the deterioration in quality of modulated signal light due to the change in characteristics of a drive signal.